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No. 0314
A Micrographic Comparison of Greases:
STRATCO® Contactor™ Reactor vs. Kettles
Presented at the 2003 NLGI Annual Meeting
Hilton Head Island, South Carolina
By
John Kay (STRATCO, Inc.), Ranjit Panesar (Ironsides Lubricants Ltd.) and David Turner (Shell Global Solutions)
NLGI Preprint: Subject to revision. Permission
to publish this paper, in full or in part, after its
presentation and with credit to the author and the
Institute, may be obtained upon request. NLGI
assumes no responsibility for the statements and
opinions advanced by contributors to its
publications. Views expressed are those of the
authors and do not necessarily represent the
official position of the National Lubricating
Grease Institute.
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A Micrographic Comparison of Greases:
STRATCO® Contactor™ Reactor vs. Kettles
Introduction
It has been very well established and reported in previous technical papers that the use of a
Contactor reactor can improve the yield of greases [1][2]. Although logic dictates that this
improvement results from structural differences in the fibrous matrix of the thickeners, such
a difference has yet to be explored photographically to any great extent. Although there
have been numerous books, articles and technical papers written, which illustrate grease
structures via electron micrographs, the authors of this paper are aware of only one
comparison study of grease structures resulting from different manufacturing methods [3],
which was limited to simple lithium grease with the samples not clearly identified as being
produced on a laboratory or commercial scale. Also, the micrographs presented in that
study were very limited in number and clarity. Therefore, the purpose of this paper is to
present micrographic comparisons of several common grease types, which are manufactured
commercially using three different types of reactors: (1) the STRATCO® Contactor™
reactor, (2) the autoclave and (3) the open kettle.
The strategy of this study is to provide micrographic comparisons of greases which were
manufactured on a commercial scale, so as to be truly representative of greases sold in the
market place. Although it was desired that the greases from the various sources be similar
in regards to the raw ingredients used, this proved to be impractical owing to the variety in
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formulations from one manufacturer to another. The study still proves to be valid if
consideration is given to the variety of raw materials. Consequently, control batches on a
lab scale are produced which do utilize identical raw ingredients so that a true “apples to
apples” comparison can be illustrated. As would be expected, structural differences can also
result from variances in temperature profiles as well as the reactor type and related means of
agitation. However, it should be recognized that the samples included in this study are
representative of typical commercial greases produced by the types of reactors mentioned
and the comparisons will prove useful in evaluating the effectiveness of the reactors.
Although the Contactor reactor has been used to manufacture most types of greases, this
study is limited to only the most common types of greases, namely lithium, lithium
complex, calcium and aluminum complex. The samples of lithium, lithium complex and
calcium were obtained from all three methods, while aluminum complex samples are
limited to the Contactor reactor and autoclave. Laboratory samples were produced of only
the lithium grease using the Contactor reactor and an open kettle. Dropping points, 60-
stroke penetration and 10,000-stroke penetrations were measured by STRATCO, Inc. and
are presented for reference. There were a few instances where values provided by the
producer differed slightly from STRATCO’s measurements and these are indicated in the
data.
Grease Sample Preparation and Micrography
Grease samples were obtained from three different manufacturers in order to obtain samples
from the different reactor types. In most cases, the samples were obtained in their basic
form, without dyes or additives. They were tested for dropping point using ASTM D 2265
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and penetration using D 217. Small amounts of these samples were sent to a laboratory for
micrographic analysis. The basic preparation included placing a thin film on an aluminum
pan, which was immersed in hexane for approximately 30 minutes. The residual thickener
was then gold sputtered for approximately two minutes to make the samples electrically
conductive. A field emission Scanning Electron Microscope was used to photograph the
samples in order to get high resolution images.
Simple Lithium Grease
It is worthwhile to note the differences in the raw ingredients used for these three sample
greases studied. The greases produced in the autoclave and open kettle utilized only 12
hydroxystearic acid (12HSA) while the grease produced in the Contactor reactor
predominantly used hydrogenated castor oil (HCO) in combination with 12HSA. Table 1
provides some additional properties of these samples.
Reactor Type Dropping Point
°°°°F (°°°°C)
Penetration
(60 stroke)
Penetration
(10K stroke)
Soap Content
(%)
Contactor
reactor
394 (201.1) 292 293 7.7 – 8.0 (9.0)
Autoclave 401 (205) 266 279 7 – 8
Open Kettle 402 (205.5) 277 287 7.6
Table 1: Sample Lithium Grease Properties
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Previous lab studies [4] have shown that greases manufactured with HCO could require
from 23% to 44% higher soap content compared to 12HSA based lithium grease in order to
attain comparable properties. Therefore, the properties shown are understandable and
expected. However, it is worthwhile to note that the soap content presented by the
manufacturer for the Contactor reactor (shown in parentheses) is based upon the total mass
of the reactants. If the basis were adjusted to eliminate the mass of the glycerol and water
produced in the reaction, the actual soap content would be reduced to between 7.7% and
8%. Very notable is the insignificant change in penetration between the 60 stroke and
10,000 stroke conditions, which suggest a very shear stable structure.
Figure 1 provides micrographic comparisons at a magnification of 50,000X and Figure 2
provides similar comparisons at a magnification of 20,000X. The images of greater
magnification are of a boundary region to provide greater detail of individual fibers. The
lesser magnification images are presented to convey a better representation of the general
fiber matrix. This latter comparison indicates that the fiber matrix in each case is formed of
long fibers with a large length to diameter ratio. Such fibers will tend to form a more
effective matrix considering that the long fiber lengths and relatively thin diameters allow
more points of contact and greater opportunities to entangle, producing a strong mesh for
retaining the lubricating oil. It also seems apparent that, for all three samples, the fibers
provide a variety of fiber thicknesses. C. J. Boner states [5] that large length to diameter
ratios contribute to shear stability while very small fibers are more effective in holding the
lubricating oil. Consequently, a structure with both of these characteristic fibers is the most
desirable. However, a close study of the micrographs indicates that the Contactor reactor
grease appears to be composed of generally thicker fibers.
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Figure 1: Lithium Grease at 50,000 X magnification
STRATCO®
Contactor™
Reactor
Open Kettle
Process
Autoclave
Process
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Figure 2: Lithium Greases at 20,000 X magnification
STRATCO®
Contactor™
Reactor
Open Kettle
Process
Autoclave
Process
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Laboratory Simple Lithium Greases
The lithium greases produced in the lab were limited to the Contactor reactor and open
kettle method and were produced from 12HSA. Table 2 presents properties of these
greases. The same 12HSA, lithium hydroxide and base oils were used in both methods.
Unexpectedly, the yield for the open kettle process is slightly greater for identical soap
contents. Although historically the Contactor reactor grease would result in the greater
yield, this particular instance resulted in the contrary. However, it is worthwhile to note the
differences in manufacturing these two batches. The Contactor reactor batch was
completely processed in 4 to 5 hours, with the cooling rate regulated at 0.6°F/minute
between 360°F and 325°F. The open kettle batch was saponified and cooled to 200°F in
about 8 hours, also with the cooling rate regulated at 0.6°F/minute between 360°F and
325°F. The following day, the batch was reheated to 190°F to achieve circulation for
milling, which all took an additional 2 to 2.5 hours. It is assumed that the additional
processing of the open kettle sample contributed to the unexpected results in yield. (The
lower 60-stroke penetration was measured at the manufacturer’s laboratory.)
Reactor Type Dropping Point
°°°°F (°°°°C)
Penetration
(60 stroke)
Penetration
(10K stroke)
Soap Content
(%)
Contactor
reactor
394 (201.1) 255 270 8.0
Open Kettle 398 (203.3) 235 - 240 245 8.0
Table 2: Lab Scale Simple Lithium Grease Properties
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Figure 3 presents micrographs of the grease produced in the Contactor reactor and Figure 4
shows the grease produced in the open kettle. Similar to the commercial scale samples, both
methods produce a well developed structure of long and relatively thin fibers with a good
variety of fiber diameters. An interesting observation is illustrated in Figure 3 concerning
the nature of these grease fibers. The thinner fibers that cross over the relatively large
diameter fiber oriented vertically in the image appear almost fused to the surface for about
180°. This clearly indicates the very limp nature of the fibers and may indicate a significant
adhesive quality of the fiber’s surface.
Figure 3: Lithium Grease made in the STRATCO®
Contactor™ Reactor (Lab Scale)
20,000 X 50,000 X
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Figure 4: Lithium Grease made in an Open Kettle (Lab Scale)
20,000 X 50,000 X
Lithium Complex Grease
All of the lithium complex greases tested are produced from the use of 12HSA. It is also
worth noting that the Contactor reactor grease utilized sabacic acid as the complexing agent,
while the other two samples utilized azelaic acid, which may have some affect on the
structure. Table 3 presents properties for these greases. Previous studies have shown that
using the Contactor reactor allows the reduction of soap content as compared to a
conventional kettle operation. As with the previous commercial Contactor reactor sample,
the soap content shown in parentheses is based upon total mass of the reactants and the
lower soap content removes the mass of the water evolved. (The lower 60-stroke
penetration for the open kettle sample was measured at the manufacturer’s laboratory.)
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Reactor Type Dropping Point
°°°°F (°°°°C)
Penetration
(60 stroke)
Penetration
(10K stroke)
Soap Content
(%)
Contactor
reactor
545 (285) 282 286 13.47 (16.0)
Autoclave 532 (277.8) 278 281 18 - 19
Open Kettle 526 (274.4) 271 - 282 293 13.87
Table 3: Sample Lithium Complex Grease Properties
Figures 5 and 6 present micrographic comparisons at 50,000X and 20,000X, respectively.
As previously, we have shown the greater magnification images near the sample boundary
to offer greater detail of individual fibers. All samples again appear to form well woven
networks of long fibers. Figure 5, which displays images from the core area of the samples,
shows a distinct difference between the autoclave grease and the other two. This distinction
appeared consistent throughout the samples. Although it is not apparent whether this
distinction is chemical, physical or both, there does seem to be a difference. The
micrographs reveal that the fibers of the autoclave grease tend to be straight, while the fibers
of the other two greases include significant quantities of fibers with twists. Also, extremely
thin fibers are much more prevalent in the autoclave sample and the Contactor reactor
sample appears to be composed of thicker fibers than the other two.
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Figure 5: Lithium Complex Greases at 50,000 X magnification
STRATCO®
Contactor™
Reactor
Open Kettle
Process
Autoclave
Process
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Figure 6: Lithium Complex Greases at 20,000 X magnification
STRATCO®
Contactor™
Reactor
Open Kettle
Process
Autoclave
Process
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Calcium Grease
The calcium greases obtained were produced with all three reactor methods. Although our
goal was to obtain samples manufactured from the same raw ingredients, we were unable to
meet this goal. In fact, the Contactor reactor and open kettle samples were manufactured
using 12HSA, which is an anhydrous calcium grease, and the autoclave grease was
manufactured from tallow, which is a hydrated calcium. Table 4 presents the properties for
these grease samples. The properties associated with these samples are as expected
according to the types of greases produced, e. g. the relative dropping points and relative
soap contents. (Please note that the unexpectedly low 60-stroke penetration of the Contactor
reactor sample is probably in error and, unfortunately, could not be reproduced since the
entire sample amount was subsequently subjected to 10,000 strokes.) Also, it is important
to note that the soap content shown for the Contactor reactor, which was provided by the
manufacturer, includes the mass of the water evolved in the process, whereas the soap
content associated with the open kettle does not. Adjusting the soap content of the
Contactor reactor grease to the same basis as the open kettle grease reduces the actual soap
content to approximately 8.7%, or 2% less than the open kettle soap content.
Reactor Type Dropping Point
°°°°F (°°°°C)
Penetration
(60 stroke)
Penetration
(10K stroke)
Soap Content
(%)
Contactor
reactor
304 (151.1) 234 (!) 262 8.7 (9.15)
Autoclave 220 (104.4) 284 291 16 - 18
Open Kettle 304 (151.1) 262 274 10.7
Table 4: Calcium Grease Properties
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Figure 7 presents micrographs of the anhydrous calcium grease produced in the Contactor
reactor and Figure 8 shows the hydrated calcium grease produced in the autoclave. These
micrographs tend to be more representative of a comparison of two different grease types
than a comparison of greases from different reactor types. (Actually, we had expected the
autoclave sample to be an anhydrous calcium grease and the fact that it was a hydrated
calcium was discovered only after we obtained the micrographs and the subsequent
dropping point measurements and questioned its chemistry.) Fortunately for our study, we
were able to obtain an open kettle sample of an anhydrous calcium grease. This open kettle
sample did incorporate the manufacturer’s standard additives, but we were hopeful that the
grease’s fiber structure and appearance would still offer a beneficial comparison. This
sample is shown micrographically in Figure 9.
With these micrographs we can establish that the fiber matrix of the anhydrous greases are
well developed and include a good variety of long thin fibers. The structure of the hydrated
calcium is comprised of relatively short fibers of a spiral form, which is typical of a
hydrated calcium grease. Comparing the two anhydrous grease samples, it again appears
that the Contactor reactor grease is composed of thicker fibers than the open kettle sample.
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Figure 7: Anhydrous Calcium Grease made in the STRATCO® Contactor™ Reactor
20,000 X 50,000 X
Figure 8: Hydrated Calcium Grease made in an Autoclave
20,000 X 50,000 X
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Figure 9: Anhydrous Calcium Grease made in an Open Kettle
20,000 X 50,000 X
Aluminum Complex Grease
Aluminum complex grease samples were obtained from operations employing the Contactor
reactor and an autoclave. Samples included both the food grade and non-food grade
formulations. Table 6 presents the properties of these grease samples. As with the other
Contactor reactor samples, the soap content provided by the manufacturer is based upon the
total mass of the reactants. Using the reaction stochiometry to eliminate the evolved
alcohol, the actual soap content is reduced to approximately 9%, which is significantly less
than that of the comparable autoclave sample.
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Reactor Type Dropping Point
°°°°F (°°°°C)
Penetration
(60 stroke)
Penetration
(10K stroke)
Soap Content
(%)
Autoclave
(food grade)
510 (265.5) 272 286 11 - 12
Contactor reactor
(non-food grade)
495 (257.2) 264 298 9.08 (10.3)
Autoclave
(non-food grade)
500(260) 219 265 11 - 12
Table 6: Aluminum Complex Grease Properties
Figure 12 presents micrographs of a Contactor reactor sample of non-food grade aluminum
complex and Figure 13 presents micrographs of an autoclave sample of the same type of
aluminum complex. The structure of the aluminum complex does not appear as a well
developed string-like fiber, but resembles tufts of cotton-like fibers. There is no clear
distinction between the two reactor types. Other methods of sample preparation were used
in order to obtain a more defined image. In addition to the sample preparation procedure
previously described, three other methods were used: (1) soaking the samples in Hexane for
5 hours, (2) soaking in Toluene for 30 minutes and (3) soaking in Hexane for 30 minutes
followed by immersion in liquid nitrogen for one minute. All samples were gold sputtered
as previously described. The fiber structures were still no more evident than with the
original standard preparation. Indeed, in all of the existing literature observed by the
authors, the appearance of the structure of this type of grease is very similar to our results.
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Figure 12: Aluminum Complex Grease made in the STRATCO® Contactor™
Reactor
20,000 X 50,000 X
Figure 13: Aluminum Complex Grease made in an Autoclave
20,000 X 50,000 X
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Next, a technique known as critical point extraction was used. In this technique, grease
samples placed on copper grids were contacted with liquid CO2 at approximately 850 psig
and 70°F. The temperature was then increased beyond the critical point. Soak time in
liquid CO2 was approximately 30 to 60 minutes. The resulting micrographs revealed some
interesting structures. Figure 14 is from the autoclave and Figure 15 is from a Contactor
reactor. These micrographs reveal a definite structure, which was nonexistent in the
micrographs of the same magnification prepared in Hexane. It leads the observer to wonder
if the Hexane technique might have destroyed the thickener chemically. If this is the case,
these micrographs may be the only true representation of aluminum complex soap in
published literature. The structure appears to be fused dough-like material. The sample
from the autoclave seems to have a smoother surface texture than that of the Contactor
reactor.
Figure 14: Aluminum Complex Grease made in an Autoclave
20,000 X 50,000 X
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Figure 15: Aluminum Complex Grease made in the STRATCO® Contactor™
Reactor
20,000 X 50,000 X
Summary and Conclusions
Regarding simple lithium greases, the micrographs indicate that well developed fiber
structures of a similar nature can be produced using all three types of batch reactors. The
commercial samples in this study revealed that the autoclave sample was composed of
generally thinner fibers than that of the other two methods while the Contactor reactor
sample was generally thickest. However, it must be recognized that the variations in
processing (e. g., maximum reaction temperature, heating rate, cooling rate,
milling/homogenizing, etc.) could affect fiber structure. Consequently, it cannot be
assumed that thinner fibers are universally produced in all autoclave processes. It can also
be said that greases manufactured on a lab scale seem to be similar to commercial products
with respect to structure and properties. Such similarities are evident with significant
differences in processing time, both on a commercial scale (3 to 4 hours vs. 5 to 6 hours vs.
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8 to 10 hours for Contactor reactor, autoclave and open kettle, respectively) and on a
laboratory scale (4 hours vs. 10 hours for Contactor reactor and open kettle).
Regarding lithium complex, the micrographs indicated that the autoclave structure was
unique, while the commercial greases produced in the Contactor reactor and open kettle
appeared similar to each other. The autoclave grease appeared to be formed predominantly
of straighter fibers compared to the other two methods, which included more fibers of a
spiral nature. The micrographs of the autoclave grease appeared to show a film-like
secondary structure, which was not apparent in the other two samples. Although this would
be assumed to be related to the complexing agent, the dropping points of the three samples
suggest clearly that all were complex greases. Additionally, as with the simple lithium
greases, the greases from all three methods indicate the formation of well-developed fiber
structures. Also, our samples revealed the Contactor reactor sample to possess generally
thicker fibers. The quality of the fiber structure is not diminished by the significant
reduction in processing time associated with the Contactor reactor.
Regarding anhydrous calcium grease, the micrographic comparison is only valid between
the Contactor reactor and the open kettle. Our samples indicate that good quality fiber
structures are produced with both of these methods. However, again the Contactor reactor
sample appears to be formed of generally thicker fibers.
With respect to aluminum complex grease, the microscopic structure does not appear as
individualized fibers. The autoclave grease has a smoother surface texture than the
Contactor reactor grease. This may offer the ability to hold a greater volume of lubricating
oil with less thickener.
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In all cases, the micrographic comparison has shown that the manufacture of greases in the
Contactor reactor, with its inherent high shear blending and rapid heat transfer and reaction
time, does not sacrifice the integrity of grease structure. In fact, the samples evaluated in
this study indicate that the use of the Contactor reactor results in generally thicker grease
fibers. This holds true even though processing time is 30% to 50% less than with the other
two methods and with a lower soap content.
Acknowledgements
We would like to express our sincerest appreciation to Ironsides Lubricants, Shell Global
Solutions, Total and Chemtool for their participation in this study.
Bibliography 1. “When We Build Our Next Grease Plant…”, S. M. Niazy, E. L. Tryson, W. A.
Graham, NLGI Spokesman, November, 1976. 2. “Operating Techniques in Soap Making”, W. A. Graham, NLGI Spokesman,
August, 1962. 3. “Comparative Studies on Thermal and Mechanical Behavior of Lithium Greases
Produced Through Different Processing Systems”, C. V. Chandrasekharan, S. Chattopadhyay, V. N. Sharma, P. M. Ozarkar & B. Ghosh, NLGI Spokesman, 1997.
4. “An Economic Evaluation of 12-Hydroxystearic Acid and Hydrogenated Castor
Oil”, S. Kinnear, K. Kranz, NLGI Spokesman, August, 1998. 5. Manufacture and Application of Lubricating Greases, C. J. Boner, Reinhold
Publishing Corporation, 1954, p.25.
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